1. Field of the Invention
The present invention relates to a polycrystalline diamond substrate, more particularly to a polycrystalline diamond substrate having a smooth surface and a process for producing such a polycrystalline diamond substrate.
2. Related Background Art
Diamond has the highest hardness among all of the solid materials which are present on the earth, is electrically an insulator and has the highest thermal conductivity within a temperature range of 30.degree. to 650.degree. C. In addition, diamond optically has an excellent light transmission characteristic in a wide wavelength range except for a certain part of an infrared region. Therefore, diamond having such outstanding characteristics is expected to be utilized in various technical or scientific fields.
Further, diamond has the highest sound velocity among all of the substances, and is stable thermally and chemically. Therefore, by utilizing such characteristics of diamond, for example, an application of diamond to devices such as surface acoustic wave (SAW) devices has been investigated. A surface acoustic wave device utilizing diamond can be used in an extremely high frequency region, e.g., by using a diamond film as a substrate, as disclosed in Japanese Laid-Open Patent Application Nos. Sho 64-62911 (No. 62911/1989) and Hei 3-198412 (No. 198412/1991).
The surface acoustic wave device using a diamond film as a substrate may be fabricated in the following manner.
First, a diamond film is formed on a substrate of Si, etc., by vapor-phase synthesis. Next, the surface of the vapor-phase synthesized diamond film is polished to be smooth. Further, the vapor-phase synthesized diamond film having the smooth surface is used as a substrate, and an electrode is formed thereon by microfabrication.
As described above, when a surface acoustic wave device using a diamond film as a substrate is intended to be fabricated, the surface of the diamond film formed by vapor-phase synthesis is polished before an electrode is formed on the diamond film.
However, since the diamond has a high hardness, polishing of the surface thereof is not sufficient in some cases, and therefore some scratches caused by the polishing can be left on the surface. Further, since the vapor-phase synthesized diamond film is polycrystalline, dropout of some particles may occur due to the polishing treatment, or an unfilled or unoccupied portion remaining in the inside of the diamond film may appear on the surface thereof as a hole due to the polishing. Therefore, in general, some pits or concavities having a dimension of up to few microns still remain on the surface of the vapor-phase synthesized diamond film even after the polishing thereof. Accordingly, in the prior art, there has been posed a problem that when a fine electrode or line having a dimension of submicron to several microns is formed on the vapor-phase synthesized diamond substrate having such a surface which is not sufficiently smooth, the fine electrode or line is liable to be cut or broken.